1. Field of the Invention
This invention relates to radiation crosslinkable polymeric compositions which exhibit moisture, heat and flame resistance and which are useful in producing insulated wire and cable as well as molded products. More particularly, it relates to a radiation crosslinkable ethylene-vinyl ester or acrylate copolymer composition having a radiation crosslinked tensile strength substantially equal to a chemically crosslinked copolymer composition of the same general composition.
2. Description of the Prior Art
One of the most important areas where fire resistant polymer compositions find use is in the electrical environment, i.e., where both insulating and fire resistant properties are sought, most especially in the area of conductor insulation. At one time, extrudable compositions available to the wire and cable art were required, for flame resistance, to contain halogenated polymers such as chlorinated polyethylene, polyvinyl chloride, chlorobutadiene, chlorinated paraffin, etc., together with antimony trioxide, both components being present in sizable quantities. Alternatively, a coating of chlorosulfonated polyethylene paint was applied to a nonflame retardant insulating compound which constituted an additional manufacturing operation.
For certain types of dry transformers, particularly high voltage transformers, a problem existed in that electrical failures occurred due to surface creepage of the organic insulating component used. The problem was solved through the addition of hydrated alumina to compositions whose organic binder consisted of butyl rubber, epoxy resins or polyester resins. However, these compositions do not possess a balance of excellent extrudability characteristics, physical and electrical properties, heat resistance and flame retardance. Such compositions are disclosed in U.S. Pat. Nos. 2,997,526-7 and 8 of Kessel et al. The described compositions for such usage have poor tensile strength, elongation and percent elongation retained after aging.
Fire retarding polymeric compositions exhibiting, inter alia, improved moisture and heat resistance consisting essentially of an intimate mixture of at least one crosslinkable polymer containing as a major component an ethylenevinyl acetate copolymer, one or more silanes and one or more hydrated inorganic fillers have found wide acceptance in the wire and cable art. Compositions such as these are disclosed in U.S. Pat. Nos. 3,832,326 and 3,922,442 of North et al. These patents disclose compositions which contain 80 to 400, preferably 125-140 weight parts of filler per 100 weight parts of polymer and 0.5 to 5.0 parts of silane per 100 parts of filler. No specific concentration range of lubricant is disclosed although 2 parts of calcium stearate per 100 parts of polymer are utilized in all of the fourteen compositions of the examples.
The prior art polymeric compositions of North et al. exhibit a unique combination, or balance, of improved physical and electrical properties together with a high degree of flame and fire retardance. These highly desirable results are achieved without the use of halogenated polymers such as polyvinyl chloride and chlorosulfonated polyethylene, thereby eliminating hydrogen chloride fumes; without carbon black, thereby permitting its use as colored insulations; without any flame retardant coatings such as are currently required, thereby eliminating an additional step in manufacturing operations when the compositions are used as, e.g., insulating compounds extruded onto a conductor; and without antimony trioxide, thereby eliminating a very expensive compound.
Such compositions find particular use as white (an inherent property) and colored uniinsulation compositions, which can be extruded over metal, e.g., copper or aluminum, conductors, to provide a single layer insulating and jacketing composition which is rated according to U.L. standards for 90.degree. C. operation, and in some cases operation at temperatures as high as 125.degree., at up to 600 volts.
The insulating compositions of North et al. have found particular utility in the insulation of switchboard wire, appliance wire, and automotive wire where a unique combination of superior electrical properties combined with resistance to the degradative effects of heat and flame are essential, and where low smoke density and non-corrosive fumes are desirable.
North et al. prefer to crosslink the ethylenevinyl acetate copolymers in their compositions by irradiation with high energy sources or through the use of chemical crosslinking agents. As has been observed with other radiation cured polymeric compositions, radiation cured compositions prepared in accordance with the disclosures of North et al. have poorer physical strength properties than their peroxide cured counterparts. The reasons for this are not fully understood although the precise nature and amount of the major and minor components in the composition are thought to be a contributing factor. Attempts to improve the physical strength of North et al.-type compositions have not met with appreciable success to date.
Two North et al.-type crosslinkable polymeric compositions as shown in the examples, particularly Example II, demonstrate this difference. One product is peroxide curable while the other is radiation curable. Several modifications were made to the peroxide curable product to produce the radiation curable counterpart. The copolymer in the radiation curable product has a higher vinyl acetate content and aluminum stearate has been substituted for the calcium stearate lubricant. Although this has improved the physical strength of the radiation cured composition over what it would have been, it is still significantly lower than the peroxide cured product.
It is an object of this invention to provide a radiation crosslinkable ethylene copolymer composition having physical strength properties in its crosslinked state which are superior to those obtained heretofore.
It is another object of this invention to provide a radiation crosslinkable ethylene copolymer composition which not only exhibits superior moisture, heat and flame resistance but also possesses superior physical strength when it is crosslinked than has been obtained heretofore.